Suppression of echoes and singing in four-wire circuits



May 28,1929. H, C, SILENT 1,714,525

SUPPRESSION OF ECHOES AND SAINGING IN FOUR-WIRE CIRCUITS Filed March 1o, 1928 INVENTORV BY awew ATTORNEY Cil 'Patented May 28,1929.

UNITED STATESv PATENT OFFICE.

HAROLD C. SILENT, OF LARCHMONT, NEW YORK, ASSIGNOR T0 AMERICAN TELEPHONE AND TELEGRAPH COMPANY, A CORPORATION OF NEW YORK.

SUPPRESSION OF ECHOES AND SINGING IN FOUR-WIRE CIRCUITS.

Application filed March 10, 1928. Serial No. 260,652.

This invention relates to two-way communication systems, and, more particularly t0 two-way telephone systems which mclude, between two-wire lines, a Jfour-wire circuit containing a radio link.

In such systems, it has been heretofore the practice to associate an amplifier-detector device with each path of the four-wlre part of the circuit. Voice waves in the transmission (or outgoing) path enter the associated amplifier-detector, and the output causes the operation of a relay or relays to place a short circuit on, or otherwise block, the reception path and also to clear the transmission path. Voice waves in the receiving path, provided this path is not blocked at the time, enter the associated amplifier-detector, and the output causes the operation of the relay or relays to block the transmission path.

In accordance with the present invention,

it is proposed to control the blocking or circuit-controlling means for both pathsv by the diii'erential operation of the polar relay, which nia-y be eii'ectively actuated by means of the detector-amplifier device associated with one path, while the ampliiier-detector device associated with the other path, through a differential control, prevents false operation of the polar relay by means of echoes or noise currents.

In its more specific Iaspect, the present 1nvention involves the use of a potential operated relay in circuit between the amplifierdetector device and the differential winding of the polar relay, together with such an arrangement of condensers and resistances in the output of the amplifier-detector device las will permit a quick building-up of the differential or protecting potential, and a comparatively long hang-over period during which the differential action may be maintained'after the impulse which determined it has ceased.

read in connection with the accompanying drawing, of which Figure 1 shows an embodiment of the invention applied to the four-wire link interconnecting one terminal of a two-way radio link to a terminal of a two-wire line; Fig. 2 of which shows -a typical hang-over arrangement; Fig. 3 of which shows, in simplified form, a type of hangshow three different forms of hang-over arrangements in accordance with the invention; Figs. 4a, 5a and 6a show the same hang over arrangements in simplified form, whi e Figs. 4b, 5b and 6b are curves illustrating the principles of the hang-over circuits.

Referring to Fig. 1, L designates an ordinary two-wire telephone line which is balanced by a network N, and, by means of the usual type of hybrid coil,is connected to a transmittingpath TL and receiving path RL of a four-wire circuit. The transmitting path TL may be arranged to lead to a radio transmitter (not shown), and the receiving path RL to lead from a radioreceiver (not shown). The transmitting path -includes a one-way repeater TR, of known type, and, likewise, the receiving path RL includes a similar one-way repeater RR.

A polar relayPR, having its windings differentially arranged, controls the circuits of relays 11 and 12, the former of which, when energized, removes a short'circuit from the transmitting path TL, and the latter of which, when energized, applies a short circuit to 'disable the path RL. An amplifier TA and a detector or rectifier TD are associated with the transmitting path in such a manner that the output current of the detector controls the energization of the lower winding of the polar relay PR. In a similar manner, an amplifier `RA and a detector arrangement, comprising tubesRD and RD', arc associated with the path RL to control the energization of the upper Winding ofthe polar relay PR. A potential operated vacuum tube relay V is interposed between such upper winding and the detector .arrangement, and the output circuit of the detector has associated therewith a condenser c, and a resistance r1, as shown, for the purpose of giving a desired hang-over action, as will be described later. The invention will b e more clearly under- I, stood from the following description when `passes through the amplifier TA to the detector tube TD, thereby causing a unidirectional current to tlovv through the lower winding of the polar relay PR. The armature of the polar relay. which normally rests upon its upper or back contact, is thereby moved to its front contact to energize the relays l1 and 12. Relay 12 disables the receiving path RL by applying a short circuit thereto, and relay 1l removes the short circuit normally applied to the transmitting path TL. A delay circuit TDC is provided in the path TL, so that the disabling short circuit controlled by the relay l1 may be removed by the time the beginning of the voice wave arrives at the point at which the short circuit was applied.

As will be observed, while the transmitting path is normally disabled, the short circuit controlled by the relay 12 is not normally applied to the receiving path RL, and consequently the system is in condition to receive at any time. lt is therefore unnecessary for a received wave incoming trom the radio receiver to actuate any circuit controlling device to condition the receiving path RL for operation, and the received voice wave will therefore be transmitted through the hybrid coil to the line L. llt the hybrid coil and balancing network could be designed so that a perfect balance would be obtained, no part of the energy of this voice wave trom the path RL would be transmitted to the path TL. In actual practice, however, an imperect condition of balance exists, so that it is necessary to guard against the possibility of noise currents, static or voice currents incoming trom the path RL being transmitted through the hybrid coil to the transmittingpath TL with sucient volume to actuate the polar relay PR through the detector fill) and thereby disable the receiving path. Accordingly; some of the energy of the received voice wave (orother wave) incoming over the path RlL is transmitted through the ampliher RA and rectified by the tubes RD and RD.

Normally, a sufficiently negative voltage is applied to the grid oit the tube V from the grid biasing battery associated with said tube to prevent any current flowing in the plate circuit thereof, and hence no current will flow in the upper winding of the polar relay PR. When rectified current Hows in the output circuit of the detector tubes RD and RD, however, such current flows through the resistance r, as soon as the capacity 01 is charged, and thus super-poses on the normal potential of the grid ot the tube V a positive potential which permits a current to ilow in the plate circuit of the tube V. rlhe eect of this current is to hold the arma ture of the polar relay PR against its back contact and prevent a false operation of the polar relay by reason ofy energy transmitted through the hybrid coil to produce a rectified current in the output circuit of the detector tube TD.

The condition above described prevents operation of the polar relay during the'eontinuance of a received wave. lllt is necessary, however, to maintain this condition a sucient time after the end of the wave has passed the point at which the amplifier RA is associated with the receiving path, to insure that the wave passing on from this point, through the hybrid coil and back into the path TL will not actuatc the polar relay PR. This hang-over period to allow the echo to die out is provided by the discharge of the condenser c, through the resistance r1. Such discharge takes place as soon as rectified current ceases to pass through RD and RD.

"lhe current discharged by the condenser is ot such a directionas to continue to superpose on the normal negative potential of the grid a positive potential, which permits a gradually decreasing current to flow in the output circuit and hold up the armature ot the polar relay PR.

Ry using the discharge of the condenser c, to maintain a guarding potential on a voltage operated relay such as V, it is unnecessary to provide a large discharge current through the resistance r1, and hence the condenser-cl may be much smaller and r, much larger than would be the case if the charge or discharge in this condenser were to be used directly to produce a hang-over current' through the winding of the polar relay PR. Ry making the condenser c1 small, the desired hang-over time can be obtained by making the resistance r1 large (since the time of discharge is determined by the product of c, and r1). Vlith the condenser o, small, the time required to charge the condenser and thereby initiate the protective action tor the relay PR at the beginning of a wave may be made very short.

ln order .to more clearly understand the advantages of the arrangement employed, in which a voltage controlled relay V is operated by means ofthe charge and discharge of a condenser through a resistance in the output circuit of the rectifier, a typical circuit in which the charge and discharge ot the condenser is used directly to provide the holdover current :tor the polar relay is illustrated in Pig. E2, the corresponding elements of the improved circuit using the voltage controlled relay being illustrated in similar relation in Rig. 3. In Pig. 3, it will be notedthat only .one detector tube RD is illustrated, it being obvious that the two tubes RD and RD, as shown in Pig. l may be replaced by a single tube where rectification of only one-halt of each alternating wave is desired. ln Rig. 3, VR designates that portion of the circuit which comprises the voltage operated relay and which is interposed between the polar relay and the detector RD with its associated condenser and resistance.

Considering the arrangement of Rig. Q, it is apparent that when no voice Waves are applied to the detector RD, the output ot the tube RD is in edect an open circuit, and the condenser c will therefore be maintained fully charged to the potential of the B battube.

flow through the protecting winding of the relay PR because the voltage across the condenser c is equal and opposite to that ot the B battery. Then voice waves are applied to the grid of the detector tube RD, however, theispace between the plate and filament becomes conductive, and the Icondenser c begins to discharge through the impedance of the tube. As the condenser discharges,v there is a decrease of the potential across the con'- denser which opposes the electroinotive force of the battery, and consequently current 'from the B battery begins to build up and flow through the holding winding of ythe poiar relay PR and the resistance 1' and thence through the plate impedance of the As only a limited amount of current can flow through the tube, it is obvious that ,the current through the holding Winding can not build up to its full value until the discharging current from the condenser c ceases. The time required for the current through the winding of the polar relay to build up toits operating value depends upon the time required to dischar e the condenser 0. For reasons which wil appear later, it is necessary to make the condenser c veiy large in this circuit arrangement, and consequently the buildingnp of the operating current in the winding of the polar relay PR may aetually require more time than that required i'or the echo to occur in which case false op-l eration of the polar relay might occur because of the echo current having had time to enter the transmitting path and energize the other winding of the polar relay before the protecting current could be built up in the output of the detector RD. l

During the continuance of the'voice wave applied to the grid of the tube RD, the protecting current. through the winding of the polar relay PR and through the resistance r continues to flow and hold the armature of the polar relay against its back contact. The condenser c having given upto the tube an rent now continues to flow from the B bat-l tery through the winding of the polar relay PR and through the resistance r to charge the condenser e.v This current continues to ilow until the condenser c resumes' its normalcharge. The time required to charge the condenser cdepends on the product of c and 1'. However, c must be made verylarge 1n order to draw, during its charging operation, sufficient current through the winding of the relay PR to hold said Winding energized during the required hang-over period. But, as

already stated, the fact that the condenser e must be large makes the time oi' the initial building-up of the protecting current through the Winding of the polar relay PR so l'ong as to cause a possible false operation.

The advantage of the arrangement in Fig. 3, which employs a voltage operated relay VR between the polar relay PR and the rectiiier RD with its condenser el" and resistance r1, will now7 be apparent. With no Voice wave applied to the grid of the tube RD, the plate circuit of the tube is in eiect an open circuit. As the B battery must charge the condenser 01 through the plate circuit of the tube, the condenser 'c1 will have no charge under these conditions, and, likewise, no current willl flow through the resistance 1'1. Vhen a voice Wave is applied to the grid of the tube RD, however, a charge begins to build up in the condenser el. At the beginning of the current How, all of the curient through the tube flows into the condenser,"

but, as the condenser charges up its counter E. M. F. opposes the flow of current and more current begins to iow through r1. When the condenser el'. reaches its maximum charge, so that no further current flows into the condenser,the current flow through the resist ance r, reaches a maximum, and this current flow continues during the continuance of the wave applied to the grid of the tube RD. The ycurrent through the resistance r1 causes a drop across said resistance, thereby superposing a positive potential upon the normal negative potential applied by the grid batl teiy to the grid of the tube V. The plate circuit of the tube therefore'becomes conducting, and a protecting current is supplied from the B battery of the tube V tothe protecting winding ofthe polar relay PR.

The time required for this plate current to reach its operating value depends upon the time required for theJ condenser clto reach its maximum charge. Since, as has already been explained, the condenser o, may be very small with thiscircuit arrangement, and still provide the necessary hang-over, the build- 'ling-up time of the current through the relay PR may be very shorty. When the voice wave applied to the detector RD ceases, and the plate circuit of the detector becomes in`effect `an open circuit, the condenser 01 discharges through the resistance 111, thereby continu# ing a superposed positive potential on the .grid until the condenser is completely disvcharged.' The time that this superposed potential continues, and hence the hang-over time of the circuit, depends upon the product of the capacity c, and resistance of the element r1. As this product may be made large enough to give the desired hang-over time,

by increasing the resistance 111 and leaving the capacity of the condenser c1 small, it is evident that the circuit of Fig. 8 will provide fthe necessary hang-over, and, at the same time, permit of a very quick building-up of the protecting current through the winding ofl the relay PR.

Various arrangements of resistances and capacities between the detector RD vand the voltage operated relay VR, to give desired characteristics, may be employed. In order to compare the cha 'acteristicspf certain of these ar 'angements, the arrangement illustrated in Figs. l and 3 is shownin simplified form in Fig. 4, and two modifications are likewise shown in simplilied form in Figs. 5 and 6.

In Fig. 5, a resistance 112 is connected in series with the parallel arrangement of resistance 111 and condenser el. So, also, in Fig. 6 the parallel arrangement of resistance 11'2 and capacity c is connected in series to the parallel arrangement comprising resistance 111 and capacity 01. In the latter case, the conductors leading to the voltage operated relay VR are connected across the terminals of the resistance 111, whereas in the arrangement of Fig. 5 these conductors are connected to the opposite terminals of the resistances 111 and In order to compare the circuit arrangements of Figs. 4, 5 and 6, the plate circuit of RD may be replaced by a resistance 11 equal to its impedance when the voice wave is applied, and a switch, which may be opened to disconnect such resistance, to simulate the condition of infinite impedance through the plate circuit oi the tube when no voice wave is applied to the grid. The equivalent circuit. arrangements are then as shown in Figs. da, 5 and Gn. rIhe various values of the voltage e applied to the grid of the voltage operated relay VR under the various operating conditions are shown by the curves of Figs. 4b, 5b and 6".

It', in Fig. 4", the switch is closed, (this corresponds to the application of a voice potential to the grid of the detector), current flows through the resistance 113 to charge the condenser c1. At the beginning of this operation, there is no voltage drop through the condenser, and the full current supplied through the resistance 11, flows into the condenser. At this instant no current flows in the parallel circuit through the resistance 111. As the condenser.) charges up, however, its voltage increases,

and hence the current through the resistance 111 wbuilds up in accordance with the wellknown exponential law until the condenser reaches its maximum charge, at which time the current through the resistance 111 will have reached its maximum value. This occurs at the time b of Fig. 4;", and the corresponding drop e through theresistance 111 will then have the value b-b. If we assume that the voltage e must build up to a value a-a in order to produce the desired protecting current through the winding of the polarrelay, it is evident that the'time o-ca will be required to build up the voltage e to the necessary value.

So long as the switch in Fig. Il remains closed (that is, so long as the voice wave is applied to the detector), the voltage e is maintained at the value b-Z1. It'. now, the switch is opened at the time in Fig. 4b, the condenser 01 must discharge through the resistance 111. This discharge current through the resistance 111 gradually decreases, and

hence the voltage e d rops oli' from the value :l2-a," in accordance with the exponential law. If it be assumed that this voltage may drop'to the value 1/ before the desired protecting current falls to too low a value (it requiring a smaller current to hold the winding of the polar relay than was necessary toenergize it in the first place), it is evident that the hang-over time will be -y.

The arrangement of Fig. 5 is such that the volt-age e will instantaneously build up to a value determined by the dropthrough 11., and will then gradually build up to a maximum as the condenser cl is charged. Referring to Fig. 5a, when the switch is closed. the condenser c1 will have Zero charge so that there will be no potential drop across its terminals, and the. resistance 111 will be in etl'ect shortcircuited. The potential difference e will therefore be equal to the drop through the resistance 11,., 4and will instantaneously rise to the value o-o of Fig. 5b. As the condenser charges up, its counter E. M. F. increases and current begins to How through the resistance 111, this current increasing to a maximum value when the condenser c1 receives its full charge. The potential e therefore increases from zero according to the exponential law until it arrives at its maximum bw.

When the switch is opened, current instantaneously ceases to ilow through the resistance 112 and therefore the potential e drops to the value -m' of Fig. 5*. Ihe discharge of the condenser c, maintains a gradually decreasing iiow of current through the resistance 111 so that the potential e drops ott in accordance with the exponential law. It a-a' be the potential necessary to eli'ectivelv energize the winding of the polar relay. and y1/-.1/" be the potential necessary to maintain it energized, the building-up time will be 0-(11, and the hang-over time --1/. It will be seen that by this arrangement the building-up time may be made somewhat shorter.

In the arrangement of Fig. 6, the condensers and rcsistances should be so designed that the charge in condenser c1 approaches its maximum more rapidly than the charge in the condenser 02. With this arrangement, the potential e will, when current begins to flow through the plate circuit of the vacuum tube, build from zero up to a maximum somewhat above the value that it has when-a state of equilibrium is reached. It will then drop off to a slightly lower value which is maintained during the state of equilibrium, and when current ceases to flow in the plate circuit of the tube it will decrease in accordance with the exponential law, as shown by the curve of Fig. 6b.

The reasons for this will be clear from the equivalent circuit'of. F ig. .6a. Here, when the switch is closed, at the instant the current begins to fiow, the condenser c, has zero charge and in effect short circuits the resistance 1', so that the potential e will'liave the value zero.

As time passes, however, the condenser c2 gradually charges causing more of the current to flow through the resistance fr', and causing the drop across 'r2 to rise. This reduces the total current in the circuit and consequently the potential e so that after a sufficient time has elapsed, e will fall to the final value v n 7'l e Efl'lrz'l'rg; By making the condenser c2 smaller than was assumed above, it becomes partially charged before c, has reached its maximum charge so the potential e increases as s hown by the curve o-a of Fig. 6b to a maximum value m in the interval o-Jmh m is'somewhat smaller than the value e of Equation (1). a then falls, approaching the final value e given by Equation (2) which is substantially attained after the additionalinterval in l) has elapsed.

By giving suitable values to the resistances r, and fr2 and to the capacities of the condensers e, and c, the height of the maximum of the potential e, its final value and the intervals ot time required to reach the maximum and the final values may be controlled.

When the current ceases to flow in the platev circuit of RD in Fig. 6, which corresponds tothe opening oftlie switch in the corresponding circuit of Fig. 6, the condenser c2 discharges through the resistance r2 without producing any effect upon the potential e. The discharge of the condenser c1 through the resistance r1, however, causes the potential e to drop off in accordance with the exponential law, as shown in Fig. 6*. An analysis of the curve of Fig. 6l" indicates that if a-a is the `potential value required to cause the winding ofthe polar relay to receive its operating current, and the potential y-fy is suflicient to provide a desired holding current,the building-up time will be o--a, and the hang-over time :x1-y. While the maximuni value ln-m2, of the potential e is not necessary from the standpoint of producing energization of the winding of the polar relay, it does have the effect of causing the potential to build up more rapidly and therefore to shorten the building-up time o-a.

It will bel obvious that the general principles hereiny disclosed mayl'be embodied in many other organizations widely different from those illustrated without departing from the spirit of the invention as defined in the following claims.

What is claimed is:

l. In a two-way communicationsystem, a first path adapted for transmisison in` one direction, a second path adapted for trans'- mission in the opposite direction, lthe first path being normally blocked and the second path being normally cleared, a two-wire line connected to said paths at one end thereof, an amplifier-detector associated with each-path, a relay having windings differentially connected and arranged to be energized under the control of said amplifier-detectors, means responsive to the operation of said'relay for clearing said first path and simultaneously blocking said second path, a voltage operated relay interposed between the output lof the amplifier-detector associated with said second path and the corresponding winding of said differential relay, and means associated with said amplifier-detector and responsive to transinisison over said second path to build up a protecting voltage to enable said voltage operated relayto prevent operation of said differential relay.

`2. In a two-way communication system, a first path adapted for transmission in one direction, a second path adapted for transmission in the opposite direction, the first path being normally blocked and the second path being normally cleared, a two-wire line connected to said lpathsat one end thereof, an amplifier-detector associated. with each path, a relay having windings differentially connected und arranged to be energized under the control of said amplifier-detectors,

means responsive to the operation of said reto transmission over said second path to build up a protecting voltage to enable said voltage operated relay to prevent operation i of said differential relay, and means to mainsion in the opposite direction, the first path being normally blocked and the second path being normally cleared, a two-wire line connected to said paths at one end thereof, an amplifier-detector associated with each path, a relay having windings differentially-conl nected and arranged to be energized under the control of said amplifier-detectors, means responsive to the operation of said relay for clearing said first path and simultaneously blocking said second path, a voltage operated relay interposed between the output of the i amplifier-detector associated with. said second path and the corresponding winding of said differential relay, and means comprising a combination of resistance and capacity associated with said amplifier-detector to build up a protecting voltage in response to transmission over said second path, said protecting voltage enabling said voltage operated relay to prevent operation of said differential relay.

4. In a two-way communication system, a

ii'rst path adapted for transmission in one direction, a second path adapted for transmis.- sion in the opposite direction` the first path being normally blocked and the second path beingnormally cleared, a two-wire line connected to said paths at one end thereof, an ainplier-detector associated with each path, a relay having windings differentially connected and arranged 'to be energized under the control of said amplier-detectors, means responsive to the operation lof said relay for clearing said first path and simultaneously blocking said second path, a voltage operated relay interposed between the output of the amplifier-detector associated with said second path and the corresponding winding of said differential relayand means comprising a combination of resistance and capacity associated with said amplifier-detector to build up a protecting voltage in response to transmission over said second path, said protecting voltage enabling said voltage operated relay to prevent operation of said differential relay, said capacity being suiiciently small to enable said volt-age to be built up quickly, and said resistance being so proportioned to said capacity as to permit said .voltage to decay sufficiently slowly to maintain said differential relay unoperated a predetermined time after transmission over said second path has ceased.

5. In a two-Way communication system, a first path adapted for transmission in one direction, a second path adapted for transmission in the opposite direction, the first path being normallyblocked and the second path being normally cleared, a' two-wire line connected to said paths at one end thereof, an amplifier-detector associated with each path, a relay having windings differentially connected and arranged to be energized under the control of said amplifier-detectors, means responsive to the operation of said relay for clearing said rst path and simultaneously blocking said secondpath, a voltage operated relay interposed between the output of the amplifier-detector associated with said second path and the corresponding winding of said differential relay, and means comprising a combination of resistance and capacity ciated with said amplifier-detector to build up a protecting voltage in response to transmission over said second path, said protecting voltage enabling said voltage operated relay to prevent 4operation of said differential relay, said capacity being sufficiently small to enable said voltage to be built up quickly, and said resistance being so proportioned to said capacity as to permit said voltage to decay sufficiently slowly to maintain saiddidcrential relay unoperated a longer time after the cessation of transmission4 over said second path than was required to build up said voltage.

In testimony whereof, l have signed my name to this .specification this 9th day of March 1928.

HAROLD C. siLnN'r. 

